I think you are being too harsh on McGrath, and unrealistic in your questioning Irwin. The use of a neutral force to keep warring sides apart is just that. As a tactic, it can and has had a high degree of success such as in Kosovo, and some spectacular failures as in Yugoslavia. It can never stop all incidents and is not intended to. That can only be achieved when both sides are prepared to give up even their most cherised demands for the sake of peace.
In the case of the Palestine / Israel conflict I can see no evidence of this. Unfortunately, it appears to me as and interested but non aligned observer that Israel is far more intransigent and beligerent than the Palestinians, and is falling into the same cant as virtually all terrorists of justifying its excesses by reference to past wrongs suffered. I am well aware that there are a large number of Israeli's working actively for peace but again, and this is my perception, if it is incorrect please clarify, because of the way the Knesset is elected, the religious and pro west bank settlement extremeists always seem to hold the balance of power.
The move by the PIRA on the other hand appears to be a truly fundamental concession in the context of Irish revolutionary / terrorist / nationalist history. Lets just hope that continues, and acknowledge the work done by the US in making this possible, if not directly at least in preparing the ground.